Does Adobe Launch Server Side support OAuth2.0 authentication flow towards external system?

The authentication in Adobe Launch Server-side (a.k.a Event Forwarding) is called as "Secrets" and OAuth2.0 is supported method. It can be configured in the UI and using API as well. API would be a preferred way for most S2S implementations. You can read more about it here: https://experienceleague.adobe.com/docs/experience-platform/tags/event-forwarding/secrets.html?lang=en